Marijuana use among Colorado teens has seen a slight decrease since the state legalized the drug, according to a report released this week by the Colorado Department of Public Health.

The Healthy Kids Colorado survey, which is conducted every two years in schools by the state government, shows that since Colorado's Marijuana Legalization Amendment was passed in 2012, teen marijuana use has dropped slightly from 22% in 2011 to 21.2% in 2015:

Colorado Department of Public Health & Environment

That's slightly lower than the national average of teen marijuana use for 2015, at 21.7%:

Colorado Department of Public Health & Environment

There was a slight increase in use between 2013 and 2015, but Larry Wolk, the executive director of the Colorado Department of Public Health and Environment, told the Denver Post that the increase from 19.7% to 21.2% of students saying they'd used marijuana in the previous month was not statistically significant enough to be a reliable sign that usage had actually gone up in those years. That suggests that the drop between 2011 and 2015 could either be a real decrease in marijuana use or a sign that the rate has stayed roughly the same.

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Either way, Wolk told the newspaper that the statistics tell us that the legalization of weed has not lead to more marijuana use among teens. The Colorado Department of Public Health & Environment echoed this sentiment in a statement to Reuters: “The survey shows marijuana use has not increased since legalization, with four of five high school students continuing to say they don't use marijuana, even occasionally."

The results of the survey serve as a challenge to critics of marijuana legalization, who have long argued that young people are more likely to use the drug if it's legal. But conservative opponents to marijuana legalization did find reason to view the report's findings with caution, pointing to the survey's voluntary nature (2015's was carried out in public schools to 17,000 students from 157 middle and high schools), and saying that teenagers who are heavy or regular users of marijuana may have been less likely to take part or admit their use in a survey.

“This is really troubling that you may be missing the most at-risk kids, systematically,” David Murray, a senior fellow at the Hudson Institute, which has opposed marijuana legalization, told the newspaper.

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Among the study's other findings, Colorado teens are smoking significantly fewer cigarettes than five years ago:

At the same time as the drop in traditional smoking, however, another CDC analysis of National Youth Tobacco Surveys from 2011–2015 found that more teens in middle schools and high schools might be vaping instead:

Centers for Disease Control and Prevention

Colorado's teens are keeping pace with that trend, too, with higher rates of vaping than cigarette smoking:

Colorado Department of Public Health & Environment

So as one bad habit sees a fall, another appears to be gaining traction.